Increase hard limit for prefetch frames to INT_MAX

Don't see why it could give issues to use large number
of frames to be prefetched, but just in case make it
unlimited hard limit and kept soft limit still at 500.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55510 954f8c5b-7b00-dc11-b283-0030488c597c